1. Field of the Invention
The present invention relates to a fuel injection control device for a multi-fuel engine and a fuel injection control method for the multi-fuel engine.
2. Discussion of the Background
In recent years, alcohol fuels have shown promise as an alternative to fossil fuels from the point of view of environmental protection. FFV's (FFV: Flexible Fuel Vehicles) capable of travelling even on an alcohol fuel mixture that is a mixture of alcohol and gasoline, in addition to travelling on just gasoline, are being developed. In addition to the calorific value and the vaporization characteristics being different compared to fuel that is 100% gasoline, an alcohol/fuel mixture has different characteristics depending on the alcohol concentration indicating a mixing ratio with respect to gasoline. This means that when an alcohol fuel mixture is used in an engine for which the use of fuel that is 100% gasoline is assumed, a controlled fuel-air ratio departs from a theoretical fuel air ratio, so that an exhaust component increases or operability changes. Regarding this kind of technological problem, technology is disclosed in Japanese Patent Publication Laid-open No. 2004-293491 for obtaining the same equivalence ratio by correcting an amount of fuel injected to an engine according to an alcohol concentration of alcohol/fuel mixture.
With an FFV, the concentration of oxygen within the exhaust gas while the vehicle is travelling is detected by an oxygen concentration sensor. Alcohol concentration within the fuel is then repeatedly learned based on the results of this detection and the amount of fuel injected is controlled based on the learning results. The learning results for the alcohol concentration are then repeatedly updated in memory. When a main switch is then turned off and then subsequently turned on again, learning results for the alcohol concentration for the previous time are read out from the memory. The amount of fuel injected can be controlled on the assumption that the fuel is of the alcohol concentration of the learned results.
With the above conventional technology, when fuel of a different alcohol concentration is supplied after the main switch is turned off, the next time the engine is started the learning results for the alcohol concentration and the actual alcohol concentration will be different.
The composition of ethanol contains oxygen atoms. The amount of oxygen per unit volume required for combustion can therefore be small compared to the combustion of gasoline. The amount of fuel injected is also increased as the alcohol concentration is increased in order to obtain the same equivalence ratio. When the actual alcohol concentration is lower than the alcohol concentration for the learned results, accidental firing occurs due to the air/fuel ratio being too rich and the load on the catalyzer therefore becomes substantial.